Teledyne LeCroy DA1855A - Manual (Page 19)

Operator’s Manual
922258-00 Rev A 13
Unbalanced source impedances can have an adverse effect on common mode rejection. For
example, a differential source with impedances of 1000 and 2000 Ω, each loaded with 1 MΩ will
have a common mode rejection ratio (CMRR) of 1000 to 1. The common mode rejection ratio can be
improved to 100,000 to 1 by using 100 MΩ input resistance.
Auto Zero
Auto Zero is a feature invoked from the Channel setup dialog when the amplifier is connected via
the ProBus interface. If the Differential Amplifier is not connected through a ProBus interface, push
either the X1 or X10 button, even if a different gain is not selected. Auto Zero momentarily sets the
input coupling to OFF and determines the offset necessary to set the output at 0 Volt. During this
process the front panel input signal to the amplifier is interrupted. When the Auto Zero cycle is
completed, the input coupling returns to its previous state. Auto Zero usually takes less than one
second to complete. This feature allows you to DC balance the DA1855A simply by pushing the GAIN
button which is already illuminated. When changing gains, the Auto Zero feature is automatically
invoked, adjusting the amplifier’s DC balance.
+ Input Coupling (AC
– OFF – DC)
In OFF mode, the input connector is disconnected from the amplifier input, and the amplifier input
is connected to ground. The AC coupling capacitor is connected between the +INPUT and ground
through 1 MΩ resistor, independent of the INPUT RESISTANCE setting. In this mode, the AC coupling
capacitor is quickly charged to the average DC input voltage. OFF mode is also referred to as
precharge mode. Precharge is particularly useful prior to selecting AC coupling when the input
voltage has a DC component in excess of 19 V. The DA1855A input coupling is set to OFF and
connected to the circuit under test. When the +INPUT is changed from OFF to AC mode, the
coupling capacitor is already charged, and the trace properly centered on the oscilloscope screen.
Additionally, the risk of tripping the input overload detector and automatically disconnecting the
input is eliminated.
In the AC mode, the +INPUT is connected through an AC coupling capacitor to the amplifier input or
the input attenuator. The coupling capacitor retains its charge when the input is switched to DC,
making it possible to return to the same circuit without the precharge time. But this also makes it
possible to discharge the coupling capacitor into another circuit under test if its DC voltage differs by
more than approximately 19 V from the voltage on the coupling capacitor.
NOTE: The discharge current from the AC coupling capacitor is limited to about 70 mA. In some
situations this could damage sensitive circuits. To avoid the inrush current transient, it is therefore
recommended that the +INPUT coupling first be changed to the OFF (precharge) when measuring a
new circuit point. This will safely recharge the AC coupling capacitor in less than 0.3 seconds.
DC and low frequencies are attenuated by the AC coupling capacitor and the input resistance. With
the ATTENUATOR set to ÷10, or set to ÷1 with the INPUT RESISTANCE
cut off (-3dB point) is approximately 1.6 Hz. When the input attenuator is set to ÷1, the INPUT